The present invention relates to flakes from multilayer iridescent films for use in paints and coatings.
Multilayer plastic films which have alternating layers of two polymers of different refractive indexes are iridescent when those individual layers are of suitable optical thickness. The iridescent color is produced by the phenomenon of light interferences. The pair of alternating polymers constitute the optical core. Usually, the integral outermost layers or skin layers are thicker than the layers in the optical core. This thicker skin may consist of one of the components in the optical core or may be a different polymer which is utilized to impart desired permanent physical, mechanical or other properties to the film. The multilayer structure is conveniently produced by a coextrusion process. Such films are described in U.S. Pat. No. Re 31,780 to Cooper, Shetty and Pinsky, and U.S. Pat. No. 5,089,318 and U.S. Pat. No. 5,451,449, both to Shetty and Cooper which are hereby incorporated by reference, and other patents.
The known laminate iridescent films have been used to prepare flakes which can be used to produce a "sparkle" appearance in a variety of applications but have not been used in coating compositions such as, for example, automotive clear coat-base coat systems because of limitations on the particle thickness and size obtainable with such known iridescent films. Such size limitations are due to the fact that the production of flakes is subject to competing requirements. The iridescent films need to be brittle so that they can be cut into flakes but at the same time, they require sufficient physical properties to permit handling on the production machinery which makes the flakes.
As a practical matter, the iridescent laminate film flakes are made by cutting the films using precision cutting machinery. The machines used to precision cut iridescent laminate films are web fed and can only take widths narrower than 5.5 inches (ca 14 cm). Therefore, the iridescent laminate films have to have sufficient physical properties so as to be able to be made as a wide web on the coextrusion equipment and then be slit into narrower webs (5.5 inches or narrower) and rewound. These narrow rolls are, subsequently, unwound and fed into the cutting machines. To satisfy competing demands, it has been found necessary that the films from which the flakes are made have a minimum thickness of 1 mil (25.4 microns) and that the shortest edge dimension of the flakes made be approximately 4 mils (101 microns) or greater.
It was thought possible to produce particles smaller than 4 mils (101 microns) from films thinner than 1 mil (25.4 microns) through a milling or grinding operation but it was found that the laminate film products currently produced are not sufficiently brittle to be milled or ground. Unfortunately, those film formulation which are brittle enough to be milled or ground cannot be coextruded because the laminate film breaks at some point during the web handling and winding process.
An iridescent laminate film product capable of being size reduced and a process to accomplish this size reduction is therefore still needed. The present invention addresses this need.